the problem with your 'return on investment' calculation is that you are not taking into account the overall impact on capacity needs that a certain number of houses
generating a set amount of electricity through solar energy will have on the profitability equations of utilities companies.
you are assuming that you'll spend a certain amount of money to generate electricity and that kwh prices will remain the same. which is incorrect. there would be a drop in demand, and price would change accordingly.
the incremental analysis would be complicated enough for a group of PHD's in Utilities Management, Electrical Engineering, Finance, Economics, etc.....
and electric cars could be practical to certain people, like low wage earners who probably have to commute to work almost every day, almost no vacation time, who wouldn't be likely to take a 1200 mile 12 day road trip for fun 3-4 times a year. it would be practical for somebody who lives 20 miles from work, has to go in 6 days a week, can't miss a day, and wouldn't miss a day b/c he can't afford it. that's a person who would have use for a cheap running, quiet, efficient electric vehicle...
these companies are marketing these things as exotic and esoteric investments to help the planet, which only very wealthy people would buy to drive around their country clubs, or TV networks trying to set their 'reality' show apart by using 'cool, trendy, and environmentally friendly' vehicles...... not really the right approach in my opinion......